The Infernal Instruments
by sprinklesonpancakes
Summary: Sebastian going to destroy the world, and all hope is lost... Unless the shadowhunters do something crazy that will probably kill them. This story is basically about the TMI shadowhunters coming to get the TID shadowhunters to help them destroy Sebastian
1. Chapter 1

**This is my new story, because I love reading these stories. Hopefully mine will work. I'm not sure about the pairings they probably will stay the same I may change them but I can't tell you now there will defiantly be either Jessa or Wessa and Malec in this story. Which ship should I use Jessa or Wessa? Anyway I hope you like it. Also should I do a bit of Clary/Will? Set after City of Lost Souls - Contains spoilers.**

**Disclaimer: I don't own the Mortal Instruments or The Infernal Devices except for Clary... Well actually no I don't own Clary *cries* But I do own this plot! I think?**

Clary jerked up straight. She looked around. When she squinted her eyes Clary could make out the features of her room in the institute. Yes, Clary got to stay at the Institute while her Mum and Luke were out on their honeymoon - as long as her room was on a different floor to Jace's.

Using the back of her hand Clary wiped the sweat off her forehead. Another bad dream. She had been having nightmares about Sebastian returning. A dim glow came from her bedside clock. 1:30am. Clary sighed. She usually wasn't able to get back to sleep after the nightmares.

The small red head turned on her bedroom light while grabbing her cosy white dressing gown and slipping on her matching soft white slippers. She rubbed her eyes a couple of times trying to adjust to the light before making her way downstairs.

On her way down to the kitchen Clary paused at Jace's room. The door was closed - strange. She shrugged before making her way down the rest of the stairs. Clary turned the corner and was blinded by sudden light. After taking a few moments to adjust, she saw Jace.

He was hunched over the table, concentration etched on his face. Paper documents covered the table and the floor nearby. He looked very stressed. Clary silently made her way over him and snaked her arms around waist, in a comforting manner.

"Clary?" He asked. His voice was strained.

"Yes?" She replied concerned. Jace turned around to face her. Clary could see how tired he looked. Jace had deep bags under his eyes, and his face had gone considerably pale. "Are you alright Jace?"

"I'm fine," He said dismissing the matter "Why are you up?"

Clary hesistated, Jace seemed to understand.

"Nightmate?" He asked. Clary nodded causing Jace's expression to go from tired to worried. Clary reached for Jace's hand and squeezed reassuringly.

"Hey I'm fine," Clary gave him a sincere smile "What are these papers for?" Jace turned back to the papers and Clary wandered up beside him. Jace sighed and ran a hand through his messy curls. He shuffled through the papers, before singling one out.

"These -" He gestured to the papers "are reports on Sebastian." Clary looked down at the papers hastily and started scanning through them.

"Do they say where he is?" Clary asked, she wasn't sure if she was happy or scared.

"Well, there has been sightings of him all over the world, but we can't be sure." Jace finished, Clary nodded taking in the information. Jace handed her a piece of paper with neat handwriting scribbled all over it. Clary looked at him both eyebrows raised, Jace just motioned for her to read.

Clary read through it twice before looking back up at her boyfriend. Her mind ticking away.

"So Sebastian is using the demons to overpower one or two Shadowhunters at once then making them drink that potion of his?"Clary asked. Jace nodded. She looked down feeling sad at the memory of Amatis, Clary suppressed those feelings and looked back up at Jace. "So we need to stop him from being able to control the demons."

"Yes," Jace agreed, "But we won't be able to do that."

"I don't see why not?" Clary said, while walking over to get herself a cup of coffee. Jace chuckled with no humour.

"First of all we wouldn't be able to get near a demon before it killed us-" Clary snorted, interrupting him mid-sentence.

"You're saying one demon will kill **all **of us?" Clary asked incredulously. Jace rolled his eyes.

"Yes I mean I'm scared that one demon will kill all of us" Jace said sarcastically. This time Clary rolled her eyes. "I actually mean that we wouldn't be able to find one demon alone, because they are all in groups 24/7." He explained. Clary turned around with her coffee in her hand while using a spoon to stir.

"I think" Clary started "That we will be fine, I mean we've killed heaps of demons at once. C'mon Jace you've killed 7 demons at once singlehandedly!" Clary regretted saying that last sentence as soon as she saw Jace's smirk. Great, now she had made Jace's ego increase... If that was possible.

"Yes but I don't think I could take out a group of 100 or so demons. Well if I had the right weapon and..." Jace started mumbling to himself about how he would individually defeat 100 demons.

"One hundred demons?!"Clary exclaimed. She slowly sat down, shocked "Well that plans out." They sat in silence thinking hard on what they could do.

* * *

When morning came Jace had passed out on the table while Clary was reading through the documents sipping her coffee every few seconds. She was the one who had bags under her eyes now. Clary had told Jace he could go to sleep last night, while they were re-reading the documents, when he could barely keep his eyes open.

Clary rummaged through the papers when something caught her eye. It was a photograph. Clary picked up the photo and stared at it intently, before something in the background made all the colour drain from her face.

In the photo was Sebastian, who appeared to be walking fast as he was slightly blurry. But what caught Clary's eye was not Sebastian but the background. He was in New York. That still wasn't the thing the scared Clary stiff.

in the corner of the picture was a girl. She was wearing black skinny jeans and thigh high boots. A black singlet with a black leather jacket on top. But it wasn't the fact that the girl was wearing shadowhunter gear. It was the fact she had pale skin, freckles and frizzy read hair tied back into a ponytail. The fact that the girl was Clary.

After recovering from shock, Clary quickly shoved the photo into her dressing gown pocket. She didn't want the others to know about it, yet.

Before Clary had a chance to do anything else Izzy shuffled into the room. It was obvious she had just woken - she was a mess! Izzy's hair stuck out everywhere and was piled on top of her head like a birds nest. Her eyes were barely open and every few seconds she would yawn, letting Clary smell her disgusting morning breath. Even though Izzy did just wake up, she still looked really pretty which Clary was slightly jealous of. Izzy rubbed her hands on her pink dressing gown before speaking.

"Morning," Izzy said before yawning again. Clary gave her a smile and waved hoping there was no evidence of shock or fright on her face.

"There's bread in the fridge," Clary informed as Isabelle searched the cupboard muttering about being hungry. Izzy put some bread into the toaster before turning to face Clary.

"You guys stayed up all night reading these documents?" Izzy asked. Clary shook her head part from answering Izzy's question, part from relief she didn't notice how scared Clary was.

"No, Jace stayed up three quarters maybe, before I finally managed to convince him to get some sleep. Me, well I had a night mare and couldn't sleep so I decided to get a glass of water when I saw Jace here and decided to help him." Clary explained. Izzy nodded understanding, they all had been having nightmares lately.

"Well what have you found then?" Izzy asked perkily. Clary felt the photo in her pocket weigh 1000 tonnes. She cleared her throat, urging her voice not to come out shaky.

"Not much really, just some sightings on Sebastian." Clary lied, she cringed inside for not telling Izzy but, it was the best thing to do. Izzy sighed and wrapped her dressing gown tighter around herself protecting her from the bitter cold.

"Sebastian's going to find us." Izzy said quietly biting her lip nervously. "He probably already knows where we are." She continued. The photo in her white dressing gown pocket felt like lead.

"Well of course he already knows where we are, he knows we're at the institute." Clary said, partially feeling less guilty about the picture. Izzy nodded, a her mouth in a tight line.

"He's going to kill us isn't us?" Izzy asked deathly quiet. Clary didn't answer she didn't say anything, she couldn't but the silence did. They both jumped at the sound of the toast popping out of the toaster.

"My-my breakfast, sorry Clary." Izzy said as she quickly turned around to butter the toast, Clary just nodded and turned to look at Jace, he looked so peaceful when he was sleeping. He had a smile on his face and was drooling onto the table. Clary sat down onto the chair next to him and played with his golden curls. She kissed his temple softly causing him to mumble under his breath, she couldn't hear most words but the one word she did hear made her heart flutter.

"Clary..." he muttered again, the smile still on his lips.

"What is that?!" The sound jerked Clary out of her dreamlike state... And also woke up Jace. Alec marched into the room holding his nose. Jace sat up and rubbed his eyes. He looked over at Clary and smiled.

"What's that smell?! Looks like Isabelle's been cooking!" Alec joked and started laughing. When he caught Izzy's glare and empty plate he gulped. Alec quickly started retreating to the door but Izzy was quicker. She stood in the doorway, blocking his path. Jace had definitely woken up and was looking between the two siblings with interest, watching to see what would happen.

"So you think my cooking is bad?" Izzy asked deathly calm, like a predator stalking it's prey.

"I uh um well you see that I.." Alec looked at Jace and silently begged for help. Jace smirked.

"Actually Izzy we know your cookings bad." Izzy sent him a glare before turning her attention back to Alec. Alec backed away a little. Any other shadowhunter would of laughed at this but they didn't know Isabelle Lightwood, she even frightened Jace sometimes. Alec send another look at Jace.

"It always smells like some has been killed when you cook... Looks even worse!" Jace said, he knew he was treading into dangerous waters but to help his parabatai. Izzy shot him another glare.

"Alec, I want an honest answer." Izzy said, Alec gulped.

"Yes your cooking is bad... But it doesn't matter because you're one of the best shadowhunters." Alec said in one mouthful. Izzy thought for a moment before giving Alec quick peck in the cheek.

"Okay, thanks!" Izzy said before bouncing out the room muttering something about fixing her hair.

"Well... that was strange," Clary said, eyeing the stunned Alec waiting for his reaction. He looked over at her and shrugged.

"The perks of being her brother I guess?" Alec answered the unasked question. Clary and Jace looked at each other. She remembered the time when she use to think Jace was her brother. It was horrible to say the least.

"Guys," Jace said alarmed "I think I know how to stop Sebastian." Jace said. Both of them stared at Jace in utter shock.

"How?!" They asked simultaneously.

"I had a dream that we got some help from the past... And I think that's the only way we can stop him, we needed them and their er powers." Jace explained. Clary and Alec looked at each other before falling into a huge fit of laughter.

"What?!" Jace asked incredulously.

"You seriously don't think we believe that do you?" Clary asked giggling.

"Yes! You're supposed to I'm your boyfriend!" Jace replied.

"C'mon Jace that doesn't have anything to with it. It's the fact that it's impossible to do that! No one can go back on time. So I don't think anyone from the past can come to the future either." Alec explained. Jace opened his mouth but closed it again thinking better of it. Clary saw how distressed he was and decided to help.

"Maybe I might be able to use my rune power?" Clary suggested, Jace's eyes lit up.

"That could work." Alec agreed.

"My plan would work, like usual." Jace added, before sending Clary a thankful look. "Do you have a rune ready?"

"Well," Clary thought hard "I actually have had a rune in my mind for a couple of days now.. We could try that, it wasn't clear before but now it is. i think it's the one." She stood up and started walking out the kitchen.

"Where are you going?" Alec asked. Clary rolled her eyes.

"You don't think I'm going back in time looking like this," she gestured to herself "I'm going to get changed."

* * *

Half an hour later Clary met Jace and Alec outside the back of the institute, her hair tied back and stele in hand. Clary was in her shadowhunting gear as well as Izzy, Alec and Jace. She had her seraph blade tucked into her belt and another one inside her boot. Beside her was Izzy in the same attire except with her gold whip wrapped around her wrist.

"Ready?" Jace asked wrapping an arm around Clary's shoulders. Izzy went to Alec and they started having an argument in all the flaws in their plan. Clary and Jace could hear snippets of their conversation.

**(A/N Izzy = **_This font _**Alec =** **This font Thanks)**

**"What... they ... kill us ...?!" **Jace stiffened slightly.

_... have ... weapons! ... better ... than them ... back in time! _Jace nodded agreeing with what he thought Izzy said.

**It ... not..., ... rune ... not** **work **Clary sucked in a breath of air. Alec was right, Clary thought. It might not work.

_It __... will _Clary was going to thank Izzy after this.

**It ... kill us **

_... might ...! _Clary inched her head slightly backward to hear their conversation better. But jerked forward when Jace patted her back softly.

"Are you ready?" He asked her concerned, Clary nodded. Biting her lip she stepped forward to the wall and place her stele on the surface and began to draw.

When she finished she opened her eyes and saw that she was now face to face with a glowing portal.

"What year?" She asked Jace. He shook his head.

"I don't know, But I know what it looks like." Jace walked up to the portal imagining what he saw in his dream and stepped through, closely followed by Clary, Alec and Izzy.

* * *

Tessa sat in the her room curled in a chair reading a book, she had quickly rubbed away her tears when she realised she was crying. Oh how she was moved by books, her brother, Nate, use to laugh at her when she did cry while reading.

Tessa set the book down on the table next to her she heard a knock on the door. Tessa quickly took a couple of breaths to even out her breathing before letting them in.

At the door stood Jem.

"Tessa." He said, Tessa smiled.

"Jem," She replied, causing a smile to tug at his lips.

"May I come in?" He asked, Tessa swung the door wide open and stepped aside as Jem entered, she closed the door behind her. Before she could comprehend what was happening, she felt Jem's lips on hers. He kissed her soft and gently. When he pulled away she gasped.

"Jem!" Tessa said still in shock.

"Sorry Tessa, I just couldn't help myself you looked so beautiful in the moonlight." Jem said embarrassed. Tessa walked up to him and kissed him softly and quickly on the cheek.

"I also wanted to tell you it was dinner." Jem said with a slight smirk. Tessa laughed and followed Jem out the door .

* * *

Tessa sat down in between Jem and Sophie. She beamed while thinking about all the amazing things her and Jem will do when they get married while everyone else chattered away happy that Mortmain was finally dead. The thought of him made Tessa cringe.

"Our child is not going to be called Buford Henry!" Charlotte said exasperated. Tessa tried to stifle a giggle but when she heard Will laugh she couldn't help but laugh along.

"Are you alright Miss Gray?" Sophie asked when she saw Tessa clutching her side in pain from laughing to hard. Tessa nodded her laughter subsiding.

"Oh you know Henry and Charlotte arguing about the Baby's name. Buford, really Henry?" She asked causing Sophie to let out a giggle.

"How's it like being a shadow hunter?"

"Oh it's the most wonderful thing you could imagine. It's like wanting something for so, so long and finally being able to have it, you know?" Sophie explain

"Yes I know," Tessa replied looking at Jem.

Suddenly there was a flash of green and there in the middle of the room were four teenagers. A small red head girl stepped out from the bunch.

"We need your help."

* * *

**What did you think? Critical ****criticism always encouraged. I hope I got everyone right. Please R&R :)**


	2. Chapter 2

**OK, thanks for reviewing :) , I have tried my hardest on this story, more than all my other stories combined so I hope you like it. Also thanks for your opinions, but I'm still slightly confuzzled. Wessa or Jessa? For this story.**

**Disclaimer: I do not own The Mortal Instruments but I do own The Infernal Devices...Not :(. **

* * *

**Will P.O.V**

"Pardon me?" Charlotte asked, standing up from the dinning table. The small girl fidgeted obviously loosing her confidence. All eyes were on her. Will did have to admit she was good looking, but not compared to Tessa. Will sent a longing look at the brown haired girl, who was - like everyone else - staring at the intruders, startled.

"I'm sorry let us introduce ourselves," said a tall girl with long, silk, raven hair and dark brown eyes. She was also pretty. The girl stepped forward and smiled, flashing pearl, white teeth. "My name is Isabelle Lightwood and this-" Will snorted.

"What?!" She asked, with a raised eyebrow.

"Oh nothing, just we already have all too many Lightworms at this institute so... you can go now." Will smirked. Tessa gasped. Isabelle, Sophie and Gabriel glared at him.

"Will!" Tessa exclaimed.

"What? I was just saying what everyone was thinking." He protested lazily.

"That was not what everyone was thinking!" Tessa countered. Jem laughed lightly.

"Dear Tessa, by now you should know what Will's like. When he says 'everyone' he means himself." Jem said with a smile. The red headed girl and Isabelle shared a look before the golden boy coughed.

"Well, while you guys have all day to chatter away, we," he gestured to himself and the other intruders "only have now to introduce ourselves." The golden boy said sounding annoyed.

"As I was saying, my name is Isabelle Lightwood," Isabelle slowed down purposefully on the word 'Lightword' sending Will a glare, "but you guys can just call me Izzy, and this is my brother Alec Lightwood." Everyone at the table shared a look. That Alec person looked very similar to Will, except with a Lightwood last name. _'How could his amazing looks go into the Lightwood family!' _Will thought outraged _'Unless...'_

Will eyed Gabriel sitting next to his sister Cecily. _'Don't you dare' _Will mouthed to Gabriel, who just smirked back and swung an arm around Cecily, who sent him confused glance._  
_

"This is Clary Morgenstern," Clary sent Izzy a glare.

"I mean Fairch- Fray, this is Clary Fray."is

"Were you going to say Fairchild?" Henry asked, looking up from his dinner. Clary laughed nervously.

"No, my name is Fray. Izzy just has bad pronunciation." Izzy sent Clary a glare, who in turn mouthed back 'sorry'. The golden boy stepped forward.

"And I'm Jace Heron-"

"Lightwood," Clary laughed nervously again, "He is Jace Lightwood." Will eyed the intruders suspiciously, what were they hiding? Charlotte walked over to the intruders.

"Well it's nice to meet you," she said with a smile, "but I have to know your business. Are you looking for some extra rooms to stay at here at the London institute?" Alec coughed.

"Excuse me, did you say London?" He asked. Charlotte nodded. "Clary!" Alec groaned.

"Sorry, I must of sent us back in time to the wrong institute."

"Back in time? What do you mean back in time?" Asked Charlotte.

"Well you see, I created a portal that was meant to send us to back in time to the New York Institute, but I guess we ended up here instead." Clary explained. Henry looked up from his plate, his face beaming.

"A portal? I was the man who created the portal." He told the group. Isabelle looked over at him, impressed. Clary nodded.

"I read about that in the shadowhunter's codex. I'm guessing you're Henry Branwell?" Clary asked. Henry nodded.

"Oh yes we must introduce ourselves." Charlotte quickly introduced everyone from the London institute.

**(A/N: I'm not going to go through with the ****whole introducing of the shadow hunters from the London Institute because it's from Will's P.O.V and he already knows who everyone is and so it wouldn't work out as well. Sorry if I just confused you)**

"Wait are you _the _Tessa Gray?" Jace asked. Tessa looked up at him startled.

"Well I am Tessa Gray, but I don't know if I'm _the _Tessa Gray you mean." Isabelle stepped forward.

"He means are you the Tessa Gray the shape shifts into other things. The only warlock able to produce babies. The-" she was cut off by Alec who motioned to the clearly embarrassed Tessa.

"Well uh yes but it's not a big deal." She said modestly.

"I have to say I am quite impressed but that's nothing compared to being the best shadow hunter ever." Jace bragged. Will shrugged.

"Second best." He corrected. "This guy here is the best." Will said pointing to himself." Before Jace could reply Clary cut in.

"That means I **did **send us to the right institute." Clary noted before pointing her tongue out at Alec.

"What do you mean right institute? Why are you here?" Gideon asked what the rest of the group was thinking. The New York shadow hunters hesitated before Jace explained.

"You see we have ran into some trouble in the future. Well future for you, present for us. There's this person, who is half shadowhunter and half demon who plans to ruin the world. Run it with a demon army. Dooming us all. And we need your help... Mainly Tessa's. Because all our plans have failed but when we read about how all of you defeated Motmain, we all decided Tessa's power is the key for us to defeat him."

The room went silent. Tessa looked very pale.

Finally Gabrielle broke the silence.

"Wait why do you need all of us? Why not just take Tessa?"

Will sent him a menacing look.

"Actually that was Jace's original plan until we figured Tessa would be more comfortable with friends and failed. Plus we need all the help we can get. So what do you say? Will you help us? Please?" Asked Clary.

"Would you mind stepping outside for a moment while we decide?" asked Charlotte not unkindly. The New York shadow hunters agreed and walked into the next room. Jace wrapped an arm around Clary protectively as they went.

"I think we should help because it could - " Charlotte was cut off by Will.

"No. Defiantly not." Tessa crossed her arms.

"And why not?" She challenged.

"Because Cecily is too young. Jem is sick. And we can't loose you again Tess, if anything went wrong and you were killed. I don't-don't know what I'd do with myself." Will explained.

"Oh Will," Tessa replied, surprised by how vulnerable he sounded.

"Will, I'll be fine and Cecily is not too young, you were doing much more dangerous things when you were her age." Jem answered. "So I think we should go."

Will slinked down in his seat with a frown on his face, but other than that nobody else argued with Jem. Cecily grinned.

"Let's go save the world...again." She shouted gleefully as she galloped up to her room to get changed into her shadow hunting gear.

**Sorry it's so short but I've been super busy and I didn't want to keep you guys waiting :)**


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